1. Field of the Invention
The present invention relates to a detector of the analog type that is usable in the field of circuits for generating electrical signals at predetermined frequencies, for example in radars and frequency synthesizers.
2. Description of the Prior Art
A phase detector is used for comparing the phase of a first electrical signal, delivered for example by a variable-frequency oscillator, with that of a second electrical signal called "reference signal", furnished for example by a fixed-frequency oscillator, so as to obtain an electrical signal whose amplitude is characteristic of the phase difference according to a known law. This difference signal is then used to modify the frequency and, therefore, the phase of the first signal, for example by applying it to the control input of the variable-frequency oscillator. This arrangement constitutes a loop for phase-locking the first signal to the second signal.
This phase lock operates only if the phase difference is less than 360.degree., i.e., when both signals have the same frequency, which in general is not the case at turn-on. This results in the necessity of combining the phase lock with an adjustment to the frequency of the phase detector.
The present invention relates to a frequency/phase detector of the analog type that is usable in a phase-locked loop in the field of circuits for generating electrical signals with predetermined frequencies, for example in radars and communications and more particularly in the frequency synthesizers using phase-locked loops.
A phase detector is used for comparing the relative phase between the electrical signal from, for example, a first variable-frequency oscillator and that of a second electrical signal called "reference signal" furnished, for example, by a fixed-frequency oscillator.
When used in a phase-locked loop, this device uses the output signal of the phase detector, suitably amplified, to control the frequency of the variable oscillator.
The whole device includes in its operation two well-known stages:
1. a frequency-homing stage when the oscillators are at different frequencies;
2. a phase-locking stage in which the oscillators are kept at the same frequency by using the error voltage that is a function of the phase difference between these oscillators to control the variable-frequency oscillator.
In certain situations, the frequency-homing range is reduced and generally limited by the bandwidth of the phase-locked loop, this when a simple phase detector without auxiliary aid is used.
To perform this frequency homing, several solutions have been proposed. For example, it has been proposed to associate with the variable-frequency oscillator a frequency-search device whose effect is inhibited when the lock of the phase-locked loop is achieved. Such a solution has the major disadvantage to require a relatively long search time. Another solution consists in associating a frequency detector and a phase detector whose output signals are summed at video frequency; in this case, however, the signal of the frequency detector can introduce noise in the signal of the phase detector, which entails the risk of degrading the performance of the latter.
There have also been proposed digital devices which have the dual behaviour of a frequency detector when they receive signals with very different frequencies and that of a phase detector when they receive signals with the same frequency. Such devices have been described, for example, in the French patent application N.degree. 86 06860 filed on May 13, 1986 by the applicant. However, these devices have currently the major disadvantage of operating in a frequency band of a few tens of megahertz only.